There are well-known master-slave communication protocols. In accordance with a master-slave communication protocol as a typical example of the master-slave communication protocols, polling is performed. Specifically, a master node transmits a header with an ID, and a slave node, which is associated with the ID of the header, transmits data as a response to a network; the set of the header and corresponding response is communicated through the network as a frame. A master node will be referred to simply as a master, and a slave node will be referred to simply as a slave.
In the master-slave communication protocol, a slave passively sends data when only receiving a corresponding one of headers sent from a master in accordance with a predetermined schedule. For the master-slave communication protocol, if an event occurs in a slave, there is a requirement to immediately inform another node of the event. In order to meet such a requirement, installation, in a master-slave communication protocol, of a mechanism has been proposed; the mechanism allows slaves to autonomously transmit data without waiting for data sending timings in accordance with schedules predetermined by masters.
For example, in LIN (Local Internet Network) protocol, as an example of these master-slave communication protocols, which is applied for in-vehicle networks, headers for event-triggered frames are prepared in addition to headers for unconditional frames. A header of an event-triggered frame causes a slave detecting the occurrence of an event to transmit, via a LIN bus, a response, and a header of an unconditional frame causes a slave identified by the header to transmit, via the LIN bus, a response.
Specifically, in the LIN protocol, when detecting the occurrence of an event, a slave is triggered to send a response using an event triggered frame (see “DETAILED DESCRIPTION OF VEHICLE NETWORK SYSTEM”, MICHIO SATO, published on Dec. 1, 2005, CQ PUBLISHING CO., LTD).
In addition, the following communication method is disclosed in Japanese Patent Application Publication NO. 2007-066111. In the communication method, pieces of output data from a master are collectively transferred via a bus to slaves in a single OUT frame, and pieces of input data from the slaves are replied in their IN frames at their non-overlappedly allocated time slots to the master via the bus. This can implement regular communications between the master and the slaves. Additionally, in the communication method, when the master identifies, in an OUT frame, one or more slaves that the master wants to check for whether there are messaging requirements therein, and the identified one or more slaves reply a response frame indicative of messaging requirements, the master transmits a messaging allowance frame to at least one slave that has replied the response, thus passing, to the at least one slave, authority to the bus. This results in that the at least one slave with the authority to the bus can transmit messages to the master. That is, the communication method disclosed in the Patent Publication allows each slave to actively transmit data to, for example, the master.